1. Field of the Invention
The present invention relates to AC to DC converters, and more particularly relates to AC to DC converters with a safety capacitor discharging apparatus.
2. Description of the Related Art
In supplying the power for electronic equipments, switching power converters are widely adopted due to the advantages of high conversion efficiency and small component size they possess.
Taking the fly-back AC-to-DC power adapter as an example, FIG. 1 shows the circuit diagram of a typical fly-back AC-to-DC power adapter. As shown in FIG. 1, the typical fly-back AC-to-DC power adapter includes a safety capacitor 101, a bleed resistor 102, an input rectification and filtering unit 103, a primary side voltage clamping snubber 104, a main transformer 105, an output rectification and filtering unit 106, a feedback network 107, an NMOS transistor 108, a resistor 109, a PWM IC 110, an auxiliary coil 111 and an auxiliary rectification and filtering unit 112.
First of all, the safety capacitor 101 is connected between two line voltages to filter out the EMI noise, and the bleed resistor 102 is used to discharge the safety capacitor 101 when the AC power is plugged off, to prevent the user from getting electric shock.
The functions of the rest parts of the adapter are briefly stated below: the input rectification and filtering unit 103 is used to generate a main input voltage VIN according to the AC power; the primary side voltage clamping snubber 104 is used to clamp the maximum primary side voltage of the main transformer 105 when the NMOS transistor 108 is off; the main transformer 105, having a primary side coupled to the main input voltage VIN and a secondary side coupled to the output rectification and filtering unit 106, is used to convert power from the AC power to the DC output Vout of the adapter; the output rectification and filtering unit 106 is used to generate a DC output voltage Vout; the feedback network 107 is used to generate a feedback signal VFB, which is coupled to the COMP pin of the PWM IC 110, according to an error signal derived from a reference voltage and the DC output voltage Vout; the NMOS transistor 108, responsive to a gating signal VG, is used to control the power conversion via the main transformer 105; the resistor 109 is used to carry a current sensing signal VCS; the PWM IC 110 is used to generate the gating signal VG according to the feedback signal VFB and the current sensing signal VCS to regulate the DC output voltage Vout at an expected level; the auxiliary coil 111 and the auxiliary rectification and filtering unit 112 are used to generate a DC supply voltage VCC for the operation of the PWM IC 110.
With the AC power plugged in, the input power is transformed through the main transformer 105 to the output through a periodic on-and-off switching of the NMOS transistor 108, which is driven by the gating signal VG generated from the PWM IC 110. Once the AC power is plugged off, the PWM IC 110 will stop operating after the DC supply voltage VCC falls below a threshold voltage, and if the adapter is under light load or empty load, the safety capacitor 101 will see only one discharging load—the bleed resistor 102—due to the fact that the main input voltage VIN at the common cathode contact of the right pair of diodes of the bridge rectifier in the input rectification and filtering unit 103 is held on the bulk capacitor in the input rectification and filtering unit 103 so it will reversely bias the right pair of diodes of the bridge rectifier and thereby shut off the conduction path to the right. According to the safety regulation for protecting users from electric shock, the charge on the safety capacitor 101 should be decreased to 37% of its original maximum value in 1 second after the AC power is plugged off. To meet this safety regulation, if the capacitance of the safety capacitor 101 is set at, for example 0.82 F, the resistance of the bleed resistor 102 will have to be restricted under 1MΩ. However, the bleed resistor 102 under this resistance restriction will consume considerable power which is likely to fail to comply with the green regulation of the adapter in standby mode, for example less 100 mW. On the other hand, if we increase the resistance of the bleed resistor 102 to reduce the power dissipation, the discharge process of the safety capacitor 101 will be prolonged and the safety regulation will be violated.
Therefore, there is a need to provide a solution capable of obeying both the safety regulation and the green regulation for AC-to-DC converters. Seeing this bottleneck, the present invention proposes a novel adapter topology with a discharging scheme to accomplish both the safety and the green requirements.